stl file import is off-centered
Depending on which mouse button you use to select things in Blender, either press W or RMB (right-click) in object mode with the object selected to open the Object Context Menu.
Then choose Geometry to Origin as shown:

This will move the object's geometry to the objects location based on the average vertex position.
To perform this type of operation more quickly, after accessing the menu, you can press OG (using the underlined letters) or 31 to choose the third element followed by the first element of the submenu.
The Object Context Menu can also be accessed by pressing ☰ aka the menu key.
If the object is still not in the center of the scene after the operation, press AltG to reset the location.